The present invention relates to a method and apparatus of controlling induction motors, according to which the primary current of an induction motor is controlled being divided into an excitation component and a torque component.
A high-response control method for induction motors can be represented by a vector control method, according to which the excitation current component and the torque current component of the primary current of an induction motor are independently controlled, making it possible to obtain speed response characteristics which are comparable with those of a d-c motor. In the vector control method, the phase of magnetic flux must be detected to control the frequency, phase and magnitude of the primary current with the phase of magnetic flux of the induction motor as a reference. To find the phase of magnetic flux, a flux detector may be attached to the induction motor. This, however, makes it difficult to use general-purpose induction motors. In practice, therefore, the slip frequency of the induction motor is calculated to instruct the phase of magnetic flux.
This method, however, presents a problem described below.
The slip frequency f.sub.s is instructed as given by the following relation, ##EQU1## where r.sub.2, l.sub.2 and l.sub.m denotes quantities that are proportional to the secondary resistance, secondary leakage reactance and excitation inductance of an induction motor, I.sub.m denotes a quantity proportional to an excitation current component of the primary current, and I.sub.2 denotes a quantity proportional to a torque current component.
An intensity of the magnetic flux usually remains constant. That is, the excitation current component I.sub.m remains constant, and the slip frequency f.sub.s is given by a relation f.sub.s =KI.sub.2 which varies in proportion to the current instruction I.sub.2 that corresponds to the torque instruction. Here, a constant K is a quantity which is concerned with the constants (r.sub.2, l.sub.2, l.sub.m . . . ) of an induction motor. Therefore, the constant K must be determined to be in agreement with the constants of the induction motor.
However, it is difficult to bring the constant K set in the control circuit into agreement with the real motor constants. Moreover, since the motor constants vary depending upon the operation conditions, it is difficult to follow the constant K set in the control circuit into agreement with the real motor constants under operation.
To solve this problem, a method has been proposed according to which the secondary resistance of an induction motor is measured or is indirectly found by calculation, to change a parameter which is related to the secondary resistance of vector calculation in response to the change in the secondary resistance r.sub.2.
In practice, however, it is difficult to measure the temperature of the rotor due to the nature of its construction. Furthermore even when such measurement is possible, it will be imprecise. Indirect measurement of temperature by calculation also is imprecise.
Moreover, the relation between the temperature and the secondary resistance r.sub.2, and the relation between the secondary resistance r.sub.2 and the intensity of the magnetic flux, differ depending upon the individual induction motors. To set such relations, therefore, the motors must be investigated beforehand a very cumbersome operation. Further, it is difficult to precisely establish the relations. In addition to the secondary resistance r.sub.2, the excitation inductance which affects the intensity of magnetic flux, undergoes a change depending upon the frequency and the temperature. Therefore, it is difficult to set the slip frequency at the optimum value, and it is not possible to obtain magnetic flux of the required intensity. Consequently, it becomes difficult to control the induction motor maintaining good response characteristics, though this should be the main feature of the vector control method.